I think thats overly optimistic. You should find that the slip percentage will be in the range of 15-20% at best. The smaller, lighter, smaller wetted surface inboards Ive dealt with tend to run closer to 15%, while the newer slalom machines tend to run closer to 20%. You may be looking at a greater slip percentage with that huge reduction trans and super steep prop. If you break 40mph you'll be doing pretty good. Ive seen a few 95-97 Prostars flirt with 50mph, but that's with the 310+ hp V8's.

As far as prop selection goes, that is going to be a major hurdle for you. If you had a gearbox closer to 1:1 (as much as 1.5:1) you could probably find somthing off the shelf that would work- but with a 2:1 and a low RPM motor, youre off the grid for standard ski boat props. You'll want to narrow down your search by looking at props that are 12-14" in diameter- you wont fit anything bigger than that on the boat.